Apparatus for biaxially stretching thermoplastic tubular film

ABSTRACT

Process and apparatus wherein an extruded seamless thermoplastic tubular film is biaxially stretched by a longitudinal drawing accompanied by an inflation or blowing for transverse stretch. After first inflating the initially extruded film tube under a counterpressure of an externally applied cooling gas in a pressure chamber, the tubular film is necked down from the pressure chamber to pass through an elongated pipe having at least a first cooling section of reduced surface contact with the tubular film, after which the tubular film is reheated for further inflation and/or longitudinal stretching and may then be subsequently tempered to achieve uniform and/or stable dimensions. The resulting inflated and biaxially stretched tube is collapsed or flattened and collected on a take-up roll in the usual manner.

United States Patent Ebert et al. [451 Mar. 21, 1972 [54] APPARATUS FORBIAXIALLY [56] References Cited RET ST CHING THERMOPLASTIC UNITED STATESPATENTS TUBULAR FILM 2,947,032 8/1960 Taylor [721 Imam Ame! Ebert; bothof 3,235,632 2/1966 Lemmeretal ..18 14 S x pertal-Barmen; Fritz Holler,Wuppertal- Langerfeld; Georg Steinhilber, Wuppertal- FOREIGN PATENTS ORAPPLlCATlONS Elberfeld Germany 1,224,471. 1966 Germany ..18/14 S [73]Assignee: J. P. Bemberg Aktiengesellschaft, Wuppertal, Germany PrimaryExaminer-H. A Kilby, Jr. Filed: I p 1970 gtgrney-Johnston, Root, 0Keeffe, Keil, Thompson & Shur- [2H Appl. No.: 43,302

' [57] ABSTRACT Related Application Dam Process and apparatus wherein anextruded seamless ther- [62] Division of Ser. No. 702,733, Feb. 2, I968,Pat. No. moplastic tubular film is biaxially stretched by a longitudinal3,544,667. drawing accompanied by an inflation or blowing for transversestretch. After first inflating the initially extruded film tube [30]Foreign Application Priority Data under a counterpressure of anexternally applied cooling gas in a pressure chamber, the tubular filmis necked down from 1967 Germany"" B 911 13 the pressure chamber to passthrough an elongated pipe hav- 1967 Germany'" B 93882 ing at least afirst cooling section of reduced surface contact I968 Germany 96138 withthe tubular film, after which the tubular film is reheated Jan. 8, 1968Germany ..B 96137 for further i fl ti n d/ l itudinal stretching and mayJan. 8, 1968 Germany B 96135 then be subsequently tempered to a hi veuniform and/or stable dimensions. The resulting inflated and biaxiallystretched [52] US. Cl "425/66, 425/326 tube is Collapsed or flattenedand collected on a takemp roll in [51] v ..B29d 7/16 the usual manner[58] Field ofSearch ..l8/1FB,1FE,1FS,14S

lab 20 18 Claims, 8 Drawing Figures Patented March 21, 1972 4Sheets-Sheet l lil'ii'f INVENTORS: ALFRED EBERT ERNST PIROT FRITZ HOLLERGEORG STEINHILBER ATT'YS Patented March 21, 1912' 4 Sheets-Sheet 2 4FIG. 3

FIG-4 s R m In E VE N B E D E Du F L A ERNST PIROT FRITZ HOLLER GEORGSTEINHILBER ATT'YS Pz iten ted March 21, 1912 3,650,644

' 4 Sheets-Sheet 5 FIGS FIG. 6 L\\ INVENTORSI ALFRED EBERT ERNST PIROTFRITZ HOLLER GEORG STEIN'HILBER BY; I

ATT'YS APPARATUS FOR BIAXIALLY STRETCHING THERMOPLASTIC TUBULAR FILM Thepresent application is a division of our earlier application, Ser. No.702,733, filed Feb. 2, 1968, now issued as US. Pat. No. 3,544,667.

This invention is concerned with an improved process and apparatus forbiaxially stretching a seamless tubular film of a thermoplastic polymerby first extruding the tube from an annular die slot or so-called ringnozzle" and then drawing the tube at a rate which is ordinarily fasterthan its speed of extrusion, e.g., through a pair of pinch rollersspaced at a distance from the point of extrusion and adapted to collapsethe tube and seal it against substantial loss of air, while injectingair into the tube to blow or inflate it at a point where it is heated topermit transverse stretching together with a longitudinal stretchimparted by drawing or providing tension in a longitudinal direction.More particularly, the invention is concerned with an improvement inprocess steps and suitable apparatus for these steps in an otherwisegenerally known process in which the extruded thermoplastic tube isfirst cooled in a pressure zone where inflation has previously beenavoided and the tube is then subsequently heated and inflated in asecond zone to provide the desired biaxial stretching. The improvementherein is directed to a number of novel variations of this stretchingprocess and the apparatus required to carry out these variations.

In general, processes are known where a tubular film is produced merelyby drawing and inflating a thermoplastic polymer as it emerges in a hotor warm stretchable state from the annular die slot of the extruder.This simple type of process, however, results only in a deformation ofthe tube rather than a desirable orientation of the molecules. In orderto achieve a biaxial stretching and orientation of the polymermolecules, it has been suggested that an initially extruded and cooledtube be passed through a heated pair of rollers and then inflated whilebeing stretched. In this case, the air used for blowing or inflation ofthe tube is introduced by means of injection between two pairs ofpressing rolls, for example with a probe or other small conduit for theair. However, this process and the required apparatus present manydifficulties and lead to losses of the film.

in order to avoid two sets or pairs of press rolls or pinch rolls andthe undesirable injection of air between such roller pairs, it ispossible to improve the biaxial stretching of the tubular film at anelevated temperature by injecting or blowing air into the tube throughan opening in the center or central portion of the annular die slot, theinflated tube subsequently being collapsed between a pair of unheatedpress or pinch rolls which may also serve to draw the film forlongitudinal stretching.

in one known process, it is considered to be advisable to avoid aninitial expansion or inflation of the film tube by having the moltenthermoplastic emerge from the annular die slot into a pressure chamberin which there is maintained a controlled counterpressure which opposesthe inflating pressure of the air within the tube. The transversestretching then takes place only in a subsequent heating chamber whereexpansion or inflation of the tube is permitted to occur undersubstantially atmospheric pressure, after the tube has passed from thepressure chamber through a pipe or tubular conduit having a highlypolished inner wall. The film tube lies against this inner wall so as topreserve the elevated pressure in the pressure chamber.

Although this application of a counterpressure on the initially extrudedtube offers certain advantages in the biaxial stretching process,difficulties frequently arise in its practical application, especiallyin the case of thermoplastic polymers which tend to be sticky or tacky.For example, the films produced at the beginning of a production runtend to exhibit physical properties which are markedly different fromthose at the end of the same production run. With a number of polymerfilms, e.g., polyvinyl chloride (PVC) which has a relatively highcontent of a plasticizer, difficulties are caused by the film stickingto the polished wall of the pipe. Another problem arises due to the factthat the biaxially stretched tube after passing through the heatingchamber tends to pulsate, with a corresponding negative influence on thestretching process and a decided impairment of film quality.

One object of the present invention is to avoid the difficulties in theprocesses and apparatus previously employed in the biaxial stretching ofextruded tubular thermoplastic films so as to provide an improved andcomparably smooth processing technique yielding a film product of highquality and uniform properties.

Another object of the invention is to provide a process and apparatusfor biaxially stretching tubular thermoplastic films which is readilyadapted to a wide variety of film-forming synthetic high molecularweight polymers and which is particularly useful with polymers such aspolyvinyl chloride, vinyl chloride copolymers, polyethylene andpolypropylene where the presence of plasticizers and/or a relatively lowmelting point of the polymer has presented extremely difficult problems.

Still another object of the invention is to provide an apparatus forbiaxially stretching the extruded tubular film whereby processconditions can be more easily adapted to the requirements of individualpolymers and adjustments can be easily made to influence the desireddimensions and properties of the blown film.

Yet another object of the invention is to provide processes and suitableapparatus which permit a stretching or orientation of the tubular filmpredominately in either the longitudinal or transverse direction.

Another object of the invention is to provide processing steps and meansto more accurately control the dimensions of the tubular film as it isbeing processed and to impart a higher dimensional uniformity andstability to the finished film product.

Thus, it has now been found in accordance with the present inventionthat these and other objects and advantages are achieved by followingthe known procedure of subjecting the film tube to transverse stretchingby controlled inflation with a gas entering the film tube centrally ofits extrusion point and to longitudinal stretching by drawing theextruded film tube through pinch rolls which also flatten the tube at aspaced distance from the point of extrusion in order to close the tubefor inflation, provided that the initially extruded tubular film isfirst inflated in an enclosed pressure zone through which a cooling gasis circulated externally of the film tube under a counterpressuresufficient to limit the inflation of the film tube, e.g., up to aboutthree times its extrusion diameter, and is then necked down from thepressure zone through an elongated tubular passageway having at least afirst stage where the tubular film is further cooled while reducing itssurface contact and adhesion to the inner wall of the passageway,subsequently reheating the film tube before it is flattened andstretching the reheated tube in at least one of its longitudinal andtransverse directions, and finally collecting the stretched andflattened film tube.

In a particularly preferred embodiment of the invention for l theproduction of a tubular film which has been strongly stretched ororiented in both its longitudinal and transverse directions, theinitially extruded film tube is slightly preinflated in the pressurezone, preferably up to a maximum of three times its initial extrusiondiameter, and is then further inflated while being heated in a secondinflation zone under a substantially atmospheric pressure, the tubularpassageway interconnecting the pressure zone with the second inflationzone and the film tube being treated in this passageway in three stagesconsisting of a. the first stage as described above where the tubularfilm is cooled,

b. a second stage where braking pressure is applied to the tubular filmthrough the wall surface of the passageway, and

c. a third stage where the tubular film is preheated prior to emergenceinto the second inflation zone, preferably while again reducing itssurface contact and adhesion to the inner wall of the passageway.

On the other hand, with only slight variations in the processing stepsand only a few if any changes in the apparatus, it is also possible toproduce a tubular film which is predominately oriented in either itslongitudinal or its transverse direction asdescribed in greater detailbelow.

In general, some preliminary testing is desirable with specificthermoplastic polymers to determine the optimum amount of inflation ofthe film tube in the pressure zone as well as other process conditions.However, in order to achieve the best results and a smooth andtrouble-free running of the tubular film throughout the process, thereare certain parameters which should be observed.

For example, it is essential for purposes of the invention to neck downthe initially inflated extruded tube as it passes from the pressure zoneor chamber into the elongated pipe or passageway in which the tube isfirst cooled and then transported for subsequent heating and stretching.In other words, the maximum or inflated diameter of the film tube in thepressure zone must be larger thanthe diameter of the elongated tubularpipe of passageway, e.g., up to approximately 10 percent larger. Ingeneral this pipe diameter will usually fall someplace between thediameter of the annular die slot and the diameter of the inflated filmtube, most advantageously such that the maximum diameter of the filmtube at this point is approximately 5 to percent greater than the pipediameter. It is also advantageous to position the interconnecting pipeor passageway sufficiently close to the face of the extrusion die sothat the length of the tube as it is preinflated in the pressure zonedoes not substantially exceed the maximum diameter of the preinflatedtube. In other words, the length of the tubular film in the pressurezone is preferably less than its maximum diameter in this zone.

It is also important to maintain and control the desired maximumdiameter of the film tube in the pressure zone so constant that it doesnot flucuate beyond a minimum amount, e.g., within limits of /3 percentand preferably not more than /2 percent. The compressed gas, e.g., air,introduced externally of the film tube in the pressure zone has a doublefunction in that' it must not only limit the diameter of the film tubebut must also cool the tube which emerges from the annular die slot at arelatively high temperature. For this reason, the necessary amounts ofair, e.g., where the extruded film tube has a large diameter and/orlarge wall thickness, can be proportionately high. In such cases, afluctuation can arise in the pressure zone which is quite harmful to thestill sensitive tubular film. Also, the intentional regulation of thepressure in response to variations in the film tube diameter then tendsto operate too slowly. It has been found that an especially satisfactorytransport or conduction of the film tube occurs and a rapid regulationof the pressure takes place so as to ensure a constant film tubediameter if the air is supplied to the pressure chamber through at leastsix separate but equally strong streams or inlet channels and iswithdrawn in the same manner through an approximately equal number ofseparate streams of equal throughput. The pressure is then regulated bysupplying additional air for the cooling air stream when the film tubediameter increases over the regulated or desired value or by exhaustingadditional air from the air stream as it leaves the pressure chamberwhen the film tube diameter decreases below the regulated value. Thisparticular improvement is likewise described in greater detailhereinafter.

The improvement in the apparatus of the invention is particularlydirected to the construction and arrangement of the pressure chamber andthe elongated pipe connected thereto. Thus, this elongated pipe shouldordinarily have a diameter which is greater than the diameter of theannular die slot, preferably up to about three times greater, but at thesame time its diameter must be slightly smaller than the maximumdiameter of the preinflated tube in the pressure chamber, and the pipeshould be arranged coaxially with the tube being drawn therethrough forpartial peripheral contact of the tube with the inner walls of the pipein at least two successive stages or sections including a. a firstsection in which the inner wall of the pipe is roughened and/or groovedto substantially reduce the area of surface contact with the tube andmeans associated with this first section to cool its inner wall, e.g., ajacket or shell through which a fluid cooling medium can be conveyed forindirect cooling, and

b. a subsequent section which is preferably roughened in the same manneras the first section and which is associated with means to heat itsinner wall.

These essential features of the invention as well as certain preferredfeatures or embodiments of the process and apparatus are explained ingreater detail with the aid of the accompanying drawing in which:

FIG. 1 is a cross-sectional and partly schematic view of one preferredembodiment of the entire apparatus, certain conventional portionsthereof being shown only in part or simply omitted, it being understoodthat the apparatus need not be arranged in vertical position as shownwith reference to the direction in which the tubular film is extrudedand drawn;

FIG. 2 is an enlarged view of a portion A of the grooved inner wallsurface of the first section of the elongated pipe shown in FIG 1;

FIG. 3 is an enlarged view of the junction B between the first andsecond sections of the elongated pipe shown in FIG.

FIG 4 is an enlarged view of the wall structure C of the second sectionof the elongated pipe shown in FIG. 1, illustrating the combined useofboth a preferred textile lining material and a permeable wallstructure for applying a braking pressure on the film tube;

FIG. 5 is a cross-sectional and partly schematic view of the upperportion of the apparatus of the invention, illustrating in greaterdetail the circulation of cooling air in the pressure chamber andautomatically regulated control means to maintain a constant film tubediameter in the pressure zone;

FIG. 6 is an enlarged and partly schematic view of the control switchactivated by contact with the film tube in the pressure chamber;

FIG. 7 is a cross-sectional and partly schematic view of anotherembodiment of the apparatus similar to FIG 1 but in which the elongatedpipe extends from the pressure chamber to about the point at which thetube starts to collapse; and

FIG. 8 is a schematic view ofa preferred apparatus arranged between thepinch rolls and a takeup device for the purpose of reducing theafter-shrinkage of the film.

Referring first to the general arrangement of the apparatus as shown inFIG. 1 which is used to impart a strong biaxial stretching in bothlongitudinal and transverse directions, the molten thermoplastic polymeris first extruded in the form of a seamless tube 1 through the usualannular die slot 2a ofa conventional screw extruder 2 so as to emergeinto a pressure chamber 3 having one or more nozzles 4 for injectingcompressed cooling air and one or more discharge openings or conduits 5so that the cooling air circulates through the pressure chamber. Boththe inlet nozzles 4 and the outlets 5 are provided with suitable controlvalves to adjust the pressure in chamber 3. An opening, bore or orifice2b located centrally within the annular slot in the face of the extruderdie 2 permits the injection of air for inflation of the tube. By meansof any suitable sensor or conventional actuating element 6 equipped withrelays to the valve controls of inlets 4 and outlets 5, thecounterpressure of the air externally of the tube in chamber 3 can becontrolled to limit the maximum diameter of the tube at point la andmaintain this diameter at a substantially constant value.

An elongated pipe of several sections as indicated by reference numeral7 extends from the pressure chamber 3 to a heating chamber 8 and thefilm tube 1 is necked down and drawn through this pipe 7 for inflationin the heating chamber. This elongated pipe preferably has a diameterwhich is up to three times larger than the diameter of the annular dieslot 20 so as to correspond to the increased diameter of the preinflatedtubular film without quite reaching this preinflated tube diameter,preferably being about 5-10 percent smaller than the maximum film tubediameter at this point.

The first pipe section 7a, which is removably mounted to the pressurechamber 3 by any conventional flange connection, preferably extendsinwardly of the pressure zone as shown in FIG 1 so that its annular rim9 receiving the slightly preinflated tube is positioned at a relativelyshort distance from the face of the extruder die 2, this distance beingadvantageously no greater than the maximum diameter of the preinflatedtube in the pressure zone 3. A cooling jacket or shell 10 is placedaround the first pipe section 7a and has suitable inlet means 10a andoutlet means 10b for circulation of a cooling fluid, e.g., water, in theannular space around the inner pipe wall 11. The tube 1 is therebycooled in the first pipe section 7a by partial peripheral contact withthe inner pipe wall 1 1.

The inner wall 11 of the first pipe section 7a has a roughened surface,e.g., by abrasion with sand jets, so that the depth of surfaceroughening is at least 30 microns, advantageously about 35 to 55microns, where there is no other alteration in the surface construction.Burrs or other sharp edges on this surface should be avoided, forexample by smoothing them off with sandpaper or any similar treatmentwhich does not reduce the required depth of roughening. Such sharp edgesotherwise have a tendency to damage the surface of the tubular film. Theterm roughened surface is employed herein (1) with its precise technicalmeaning as a fine pattern of surface irregularities as may result fromthe initial casting of the pipe section or by shaping, sand blasting,turning or grinding processes, but also (2) with its general meaning ofany surface which has even larger interruptions or gap spaces recessedtherein, e.g., as obtained by a regular pattern of grooves, notches,serrations or the like. In either case, however, it is desirable toavoid sharp edges or corners which may damage the film.

In general, it is desirable toprovide a surface roughness which reducesdirect contact of the tubular film with the inner wall 11 by asubstantial amount, e.g., by at least one-tenth and' preferably at leastone-quarter to one-half or even more. The tube 1 thus tends to ride onlyon the high" portions of the roughened wall surface or can even belifted slightly from the wall surface by means of a thin layer of air asexplained more fully below.

A particularly advantageous embodiment of this first pipe section 7a isillustrated in FIG 2 where an enlarged view of portion A of the innerwall 11 is shown for greater detail. Thus, series of crossed spiralgrooves 12 can be cut into this inner wall so as to extend over theentire length of the first pipe section. These grooves, for example, mayhave dimensions of 8 mm. in width and 0.6 mm. deep. The remainingportion of the wall can thereby be cut by the grooves into rhombicsurfaces 13 whose sides measure about 3 to 8 mm. by way of example. FIG.2 merely represents such spiral grooves spaced 8 mm. apart and havingsaid dimensions of 8 X 0.6 mm. Other patterns and dimensions of thegrooves or similar notched or cut recesses are also feasible, but it hasproven to be especially advantageous if such grooves provide a pluralityof fluid channels along the inner wall from the pressure chamber 3 tothe opposite end of the first pipe section 7a.

With such grooves 12, a small amount of air can be permitted to flowtherethrough between the tubular film 1 and the inner wall 11 so as toslightly depress the. film tube inwardly away from the wall. The amountof air flowing in the grooves or channels 12 is regulated as shown ingreater detail in FIG. 3 which is an enlarged view of the terminal endof the first pipe section 7a. Thus, by providing an annular recess orgroove 14 at the end of this first section in fluid communication withthe spiral grooves 12, air can be withdrawn through one or more outlets15 having adjustable valves 16. This grooved construction in combinationwith a controlled flow of air around the tubular film permits theungrooved wall surfaces 13 to have a surface roughness of even less than30 microns. Of course, with only a few grooves or widely spaced grooves,it is again advisable to increase surface roughness, e.g., by abrasion,to values above 30 microns. By employing both a surface roughening andgrooving of the inner wall of section 7a, the flow of air around thetubular film is especially uniform and easily controlled.

The second section 7b of the interconnecting pipe can be constructed invarious ways to exert a braking pressure or frictional drag on thetubular film as described more fully below. A particularly desirableembodiment of this section is illustrated by the enlargedcross-sectional view of the inner wall in FIG 4. The outermost portion17 of this inner wall consists of a porous material, e.g., sinteredmetal, or some other air-permeable structure, e.g., a perforated metalshell, which is then lined with a textile material in the form of afabric backing 18 having a raised or napped inner surface 19. As furthershown in FIG. 1, the inner wall 20 is encased by a jacket 21 to providean annular space which can be placed under a vacuum by connecting line22. The vacuum is spread evenly by the plush textile lining which inturn exerts the desired braking pressure or longitudinal tension on thetubular film.

The third pipe section 7c has an inner wall 23 encased by a jacket orshell 24 with inlet 25a and outlet 25b for circulation of a heatingfluid around the inner wall. The surface roughening of the inner wall 23to a depth of less than 30 microns is sufficient in this case, becausethe tubular film has been cooled and braked in the previous sections andpreheating in this third section can be controlled to avoid anysoftening of the film.

A heating chamber 8 is mounted to enclose the tube I as it emerges fromthe last pipe section 70. Heating can be accomplished with conventionalmeans, e.g., by infrared rays from heating elements 26 and 27. Ingeneral, it is desirable to maintain a higher temperature in the upperportion of the heating chamber with elements 26 as compared to the lowerportion. The heating chamber is normally maintained under atmosphericpressure so that the tube 1 is inflated to achieve the requiredtransverse stretching.

A calibrating sleeve 28 with shell 29 is preferably positioned aroundthe inflated tube after the heating chamber, and the inlet 30 and outlet31 permit either a heating or cooling medium to be circulated in theshell 29 in order to temper the film tube and ensure a uniform diameterand/or uniform properties of the stretched film.

Conventional apparatus is then provided to collapse or flatten the fullyinflated tube, e.g., by using a series of paired rollers 32 driven oneach side by an endless belt or band 33. The flattening of the tube iscompleted by a pair of pinch rolls 34 which close the tube against theair pressure necessary for inflation and which also exert the requiredpull or draw to stretch the tube in its longitudinal direction. The flattube is then collected on the usual takeup spool 35 or similar windingdevice.

In order to avoid so-called butt rings in the finished film product, itis conventional to mount the takeup spool 35, the roller pairs 32 and 34with their belt drive 33 and preferably also the calibrating sleeve 28so that they can be turned slowly back and forth about the longitudinalaxis of the film tube. This same effect can also be achieved, however,if the blowing head or extruder die with the annular slot executes thisback and forth movement or revolves slowly about the longitudinal axisof the film tube while the rest of the apparatus remains stationary. Inthis latter case, the pressure chamber must be attached to the extruderby any suitable rotatable and pressuretight connection, e.g., by meansof a short cylindrical extension beyond the face of the extruder diejoined to the upper wall of the pressure chamber by a packed bearing asin the construction of a so-called stuffing box.

The individual sections of the pipe 7 as well as the heating chamber 8are advantageously joined together by easily separable flanges asindicated in FIG. 1, so that individual members can be interchanged toaccommodate different thermoplastic materials and/or differentdimensions as to the desired diameter and length of the tube beingformed and stretched.

It will be evident from the working examples below that differentthermoplastic polymers such as polyvinyl chloride and polypropylene havedistinct properties which require corresponding variations in thedimensions of the apparatus, for example with reference to bothdiameters and lengths of individual elements of the apparatus. Furthervariations in dimensions depend upon such factors as the final diameterof the tubular film, its wall thickness and also the amount of polymerextruded per unit time. Optimum dimensions can be readily determined bysimple trial experiments since the properties and behavior offilm-forming polymers under biaxial stretching are well-known withrespect to the amount of stretch, temperature of the film duringstretching, standard film thicknesses and the like. Furthermore, the airpressures applied in the apparatus, the rate of cooling or heating ofthe film and the rate of extrusion and drawing can be easily manipulatedduring operation of the apparatus.

The ratio of the diameter of the elongated pipe 7 to the diameter of theannular die 2a is generally not critical. A smaller ratio of thesediameters is advantageously chosen for thermoplastic materials having ahigh stretch factor as compared to those materials having a low stretchfactor. The term stretch factor refers to the numerical value whichspecifies how many times the initially extruded thermoplastic film canbe stretched with reference to its original unstretched length. By wayof example, an especially favorable ratio for polypropylene (which has astretch factor of about 6) has been found to be 1.5:1, while the bestratio for polyvinyl chloride (which has a stretch factor of about 2) hasbeen found to be about 2:1. At a very small ratio of 1:1, e.g., withpolypropylene, it is still possible to produce good tubular films, but acertain fluctuation appears in the tubular film which are noticeable inthe way in which the tube moves. Also, with too large a ratio of over3:], e.g., with polyvinyl chloride, fluctuations are occasionallyobserved in the form of pulsations. Even though these fluctuations haveno real influence on the process and do not diminish the quality of theresulting film, one usually attempts to achieve a smooth running of theblown film or foil, at least as some indication that the equipment isfunctioning properly. It has been established that such fluctuations arecompletely avoided if the ratio of the diameter of the elongated pipe tothe diameter of the annular die slot lies between about 1.221 and 3:1,so that this range is deemed to be advantageous.

Merely by way of example, the following data can be given with theunderstanding that smaller and larger dimensions may also be used inappropriate cases:

about 800 mm,

about 400 mm.

Diameter of heating chamber Length of heating chamber Diameter ofcalibrating sleeve Length of calibrating sleeve The exact dimensions ofthe calibrating sleeve naturally depend upon the fully inflated diameterof the tubular film, i.e., the desired amount of transverse stretch, andcan therefore be varied widely from the single values given above. Thedimensions of the other elements of the apparatus are generally validfor polymers having a relatively wide range of stretch factors.

In conjunction with the foregoing description of the apparatus, theprocess of the invention can be explained in greater detail as follows.It is again expressly stressed that numerical data is merely set forthby way of example because the method of treatment will vary withdifferent thermoplastic materials, extrusion amounts and speeds, drawrates and the like as well as with the desired properties of the finalfilm product.

It is essential for purposes of the invention to preinflate theinitially extruded hot tubular film 1 to a diameter which is slightlylarger than the diameter of the pipe 7 and to control this preinflateddiameter by means of the counterpressure in the pressure chamber 3within relatively narrow limits, e.g., :2 percent, Otherwise, if thefilm tube acquires a preinflated diameter under the counteraction of thecompressed air in chamber 3 which is equal to that of the diameter ofpipe 7, e.g., as has occurred in known processes, there exists thedanger that a relatively large amount of air will suddenly pass betweenthe film tube and the pipe walls. A large loss of product would thenoccur, and it would be necessary to reinitiate the entire productionrun. On the other hand, the diameter of the pipe 7 must closelyapproximate the maximum film tube diameter so that the necking down ofthe tube 1 into the pipe 7 is not so excessive as to cause folds orother undesirable film deformation. Thus, the maximum diameter of thepreinflated film tube in the pressure chamber is advantageously about5-l0 percent greater than the diameter of pipe 7. With this preinflationof the tube to a diameter slightly larger than the elongated pipemember, the film tube is brought into close surface contact with theentry to the first section 7a of the pipe so as to prevent anysubstantial loss or escape of air from the pressure chamber 3. It isespecially advantageous if the first pipe section 7a extends or projectslongitudinally into the pressure chamber 3 and presents an entry rim oredge 9 which is tapered or rounded radially inwardly to the requiredpipe diameter, thereby providing a smooth reception of the preinflatedtube. The radial stability of the drawn tube is enhanced by positioningthe receiving edge 9 of the first pipe section 7a at a distance from theface of the extruder 2 not more than the maximum diameter of thepreinflated tube 1.

It is also important to maintain the maximum diameter of the tube atpoint la so constant that it does not fluctuate beyond a minimum amount,e.g., preferably. within limits of :2 percent. A greater fluctuationwould of course tend to increase the possibility of breaking the airseal of the pressure chamber unless the tube were to be preinflated to amuch larger maximum diameter with reference to the receiving pipesection 7:: to accommodate such large fluctuations, in which case thedanger of film deformation would also increase substantially. Suchdesired control of the preinflated tube diameter is readily achieved bythe use of radially positioned sensors 6 or similar actuating elementsresponsive to variations in tube diameter so as to increase or decreasethe counterpressure of air in pressure chamber 3.

In the first zone or pipe section 7a, the tubular film is cooled sincethis is essential for the improvement of the process. Cooling issufficient to lower the temperature of the film at least below itssoftening range, i.e., so that the film is in the solid state and willtend to resist expansion until subsequently reheated. It is of coursepossible to impart a certain amount of precooling to the film by thecompressed air injected into the pressure chamber 3 or by withdrawingheat from the molten thermoplastic material as it emerges from theextruder die. However, such measures should not be so strong as tointerfere with the necessary preinflation of the extruded tube in thepressure chamber.

In spite of the cooling of the film tube in the first pipe section 7a bymeans of indirect heat transfer with a circulated cooling medium inshell 10, there still exists the danger that the film will stronglyadhere to the inner wall of the pipe if one were to use a polished orsmooth wall of the pipe. Thus, even a hardened or solid film tube iscapable of adhering to a smooth surface just as two plates of glass canadhere to one another. Although wall adhesion can never be completelyavoided, it is considered to be essential at this point of the biaxialstretching process to reduce such wall adhesion as much as possible.Accordingly, in combination with other features of the process, it hasbeen found that a trouble-free tubular film production and stretching ispossible provided that the inner wall of the first pipe zone or section7a is roughened to a depth of at least 30 microns, preferably 40 micronsor even more. The improvement which is achieved by this minimumroughening depth according to the invention can be explained in terms ofthe fact that some air from the pressure chamber 3 is permitted topenetrate between the pipe wall and the film tube so as to slightlydepress the tube away from the wall, i.e., so that the tube rides on alayer of air rather than on the walls of the tube.

Especially good results have been achieved by permitting a controlledamount of air to pass from the pressure chamber or zone 3 to the annularrecess 14 through the crossing spiral channels or grooves 12. In thiscase, the depth of surface roughening produced on the wall portions 13by abrasion or the like can be reduced to about 30 microns or less,preferably to a depth of -20 microns.

The tubular film is placed under a braking pressure or frictional dragon the second pipe section 7b in order to achieve an exact longitudinalstretching. The film tube is neither heated nor cooled in this secondpipe section. In order to provide a peripheral braking effect withoutcausing damage to the tube, it is especially desirable to bring theouter wall surface of the tube into contact with a soft inner lining ofthe second pipe section in the form of a textile product, particularly aplush fabric with its raised, piled or napped surface facing inwardlyfor contact with the film tube. As indicated in FIG. 4, the base fabric18 may be made of cotton while the pile 19 is composed of a polyesterfiber (polyethylene terephthalate). Both the base fabric and the pilecan consist entirely of such polyester fibers, or any other fibrousmaterial including mixtures or various combination of fibers can be usedsince the cooled film tube has no tendency to stick to a textilematerial during this braking operation. Polyester fibers in at least theinner pile surface of the fabric liner are preferred because of theirexcellent stability. Also, by using a relatively stifi cut pileconsisting of synthetic individual filaments of non-staple fibers, oneachieves the greatest frictional resistance against the longitudinalmovement of the tubular film and there are no staple or cut fibers whichcan be dislodged and carried along with the tubular film. The fabricliner should be applied to the outer cylindrical pipe structure asuniformly as possible, e.g., with any suitable adhesive at selectedpoints or preferably by stretching the fabric and clamping it in place.

Another advantageous construction of the second pipe section 7b isachieved by perforating the inner wall 20 of this section or zone asshown in FIG. 1 and then adding ajacket or shell 21 in such a mannerthat the cylindrical interspace between the wall 20 and jacket 21 can beplaced under a vacuum, e.g., through line 22. The holes or openings inthe inner wall 20 should be small but numerous. The desired permeabilityof this inner wall 20 can also be achieved by using a pipe of sinteredmetal or any other air-permeable substance. The tubular film can lightlycontact this permeable pipe wall structure itself, but it is especiallyadvantageous to also line the permeable wall with the above mentionedtextile material. This liner is especially useful in this case toachieve a very uniform vacuum distribution.

The braking force applied in the second pipe zone 7b can be adjusted bythe level of the applied vacuum, e.g., from just a few millimeters toseveral centimeters of water column, as desired. The amount of vacuum isgenerally dependent on the type of thermoplastic polymer being treated,the thickness of the film tube and its drawing-off speed. Thisadditional braking by means of a vacuum increases longitudinal shrinkagevalues by 10 to 20 percent.

lnthe third pipe section or zone 70, there is a diminished tendency forthe tubular film to adhere to the inner wall of the pipe so that itsroughening depth can be reduced below 30 microns, e.g., to a depth of5-20 microns. This third zone serves to preheat the film tube by meansof any suitable fluid heating medium circulated through the jacket orshell 24. This preheating is preferably carried out so that the filmtube attains a temperature upon its exit from the third zone whereby themost favorable shape or configuration of the tube takes place duringbiaxial stretching in the heating chamber 8. This shape or configurationunder inflation of the tube corresponds to a short compact peartruncated at either end, e.g., substantially as shown in FIG 1. If thetemperature achieved by preheating is too low, the film tube enters theheating chamber with too small a diameter and the tube then tends topulsate. Too high a temperature is harmful because the longitudinalstretch values drop.

Again, the correct temperature for the third pipe section or preheatingzone 70 and the temperatures of the heating chamber 8 will varydepending upon the particular ther moplastic material, the wallthickness of the tubular film and the drawing-off speed. Thesetemperatures can be set approximately from prior experience and generalknowledge of biaxial stretching temperatures with a minimum of testingto establish optimum results. For example, the temperature in at leastthe upper portion of the heating chamber 8 must fall within the biaxialstretching temperature of the thermoplastic polymer being treated whileit is preferable to maintain the temperature in the preheating zone andthe lower portion of the heating chamber below this biaxial stretchingtemperature. However, in some cases it is advantageous to maintain allthree temperatures within the biaxial stretching range especially withhigher melting point polymers or those which have less tendency tobecome sticky in their stretching. The range of this biaxial stretchingtemperature, within which the ther- 'moplastic material can be stretchedfor both longitudinal and transverse molecular orientation, isdetermined by means of the so-called Kofler bench" as described in thebook entitled Thermomikromethoden 1954 page 33, by L. and A. Kofier.With this technique, there were found for example the following ranges:for polyvinyl chloride (with a K-value of 70 and a content ofplasticizer of 20 percent by wt), a range of -l30 C.: for a copolymer ofpercent vinyl chloride and 10 percent vinyl acetate (with a K-value of60), a range of 8 5l3( C.; for a low-pressure polyethylene, a range of ll0140 C.; and for polypropylene, a range of l 104 50 6.

Then, in accordance with the invention, the tubular film is preferablyconducted directly from the heating chamber 8 through the calibratingsleeve 28 where the film tube runs in surface contact with the innerwall of the sleeve. By circulating a fluid heat exchange medium in thejacket or shell 29 of this sleeve, it can be maintained at a particulartemperature which again depends upon the particular thermoplasticmaterial. In the case of the thin films, e.g., 15 microns, andplasticized shrinkage films, the sleeve is cooled. On the other hand, inthe case of thicker and especially hard films such as the copolymers ofvinyl chloride, the sleeve must be heated, e.g., to about 3050 C., inorder to avoid the development of folds. The proper temperature in anyparticular case can be readily determined by routine tests or simply byobserving the film as it emerges from the sleeve.

After the fully blown and tempered film tube leaves the sleeve 28, it iscollapsed and wound on a takeup reel 35 in known manner, preferably bygradually flattening the tube over a series of oppositely paired rollers32 down to the usual pinch rolls 34. These pinch rolls are positivelydriven in order A to draw the tubular film through the apparatus at apredetermined rate.

The foregoing embodiment of the process, described in con junction withFIGS. 1-4, provides a basic explanation of the overall process andapparatus which can be employed to produce tubular films which exhibit ahigh degree of stretch both in the longitudinal or axial direction andalso in the transverse or radial direction. For various purposes,however, it is sometimes quite desirable to obtain a stretching, i.e.,an orientation of the film, which is relatively slight in one directionbut high in the other direction. It is a special advantage of theinvention that one can achieve such preferential or predominatestretching by making only slight changes in the process steps and/or byreassembling the apparatus in a relatively easy manner.

However, regardless of the particular technique employed for heating andstretching the tubular film, it has been found that particularly goodresults can be achieved in all cases by means of the apparatus shown inFIGS. 5 and 6 for the accurate control of the maximum film tube diameterin the pressure chamber. This apparatus is especially desirable whenworking with film tubes of relatively large diameter and/or thick walls,and it has a decided value in maintaining the desired flow of coolingair around the inflated film tube in the pressure chamber while stillpermitting a rapid adjustment of the external counterpressure in thepressure chamber in response to fluctuations in the film tube diameter.

As shown in FIG. 5, the screw extruder 36 is fitted with a conventionalannular die plate 37 which contains the annular die slot with a centralopening for the inflating or blowing gas introduced through theintermediate blower head 38, all in the same manner as indicated inFIG. 1. Likewise, the upper end of the elongated pipe 39 is essentiallythe same as described above. The significant features in the apparatusof FIG. are the various lines or conduits which circulate the compressedcooling air and the devices employed to regulate this flow of air.

The initial supply of cooling air enters through line 40 where itnormally proceeds through branch line 40a directly to an intermediatestorage chamber 41, from which it flows out in streams of equalproportions through a plurality of outlets 42, each of which carries along conduit 43 running to a corresponding nozzle 44 where the air isinjected into the pressure chamber 3. Only one connecting line orconduit 43 has been shown in FIG. 5 for purposes of simplicity, it beingunderstood that each nozzle or entry opening 44 is connected to one ofthe outlets 42 and that all of the lines 43 have the samecross-sectional dimensions so as to obtain approximately the same flowrate or input through each nozzle 44. Around the upper end of thepressure chamber 3, there is preferably arranged an air distributor ringor manifold 45 having a ringshaped baffle screen 46 through which theair is blown radially inwardly and is then deflected downwardly by theannular gap 47 onto the film tube 1. The number of storage chamberoutlets 42, lines 43 and nozzles 44 should each amount to at least six.Their actual number is dependent upon the diameter of the pressurechamber 3, since the distance between two adjacent nozzles 44 along thering-shaped distributor 45 should not be greater than about 30 cm. Forexample, with a diameter of the pressure chamber of about 700 mm., theentry nozzles 44 with a diameter of about mm. would lie on a circle ofapproximately 650 mm. and six nozzles would not then be sufficient.Therefore, in this particular case, eight nozzles 44 are employed whichthen have an interval between each other of about 26 cm. The cooling airleaves the pressure chamber 3 through exit outlets 48 which are equal innumber to the nozzles 44. Also, it is again preferable to use an annulargap 49 leading radially outwardly into a collecting ring 50(corresponding to the construction of gap 47 and distributor ring 45).The air then flows in equal proportions through line 51 and the shortentry tubes 52 into the storage chamber 53. At this point the coolinggas is exhausted through exit line 54 which is branched into one line54a exhausting directly to the atmosphere and a second line 54b whichcan be used for exhausting additional air.

In the pressure chamber 3, there is located a switch 55 fitted into acapsule or small fixed housing 56 in such a manner that it can becontinuously washed with small amounts of air, thereby avoiding anyfouling of the electrical contacts of the switch over a period of time.Thus air is introduced into the housing 56 through the pipe 57 (see bothFIGS. 5 and 6) and exits through slot'58. The lever arm 59 carrying thefreely rotatable pin feeler or sensor 60 enters the housing 56 throughslot 22 and is pivoted together with contact arm 61 on the pivot point62. The feeler or sensor 60 rests on the tube 1 at its maximum diameter,so that when the film tube diameter exceeds or falls below the idealdiameter, the electrical contacts 63 or 64 are either closed or openedby the contact arm 61, thereby closing or opening magnetic or solenoidvalves through a suitable electric circuit represented by lines x, y andWith this arrangement, it is possible to closely and accurately regulatethe counterpressure of the cooling air in pressure chamber 3 as can beexplained in connection with FIGS. 5 and 6. Thus, if the diameter of thefilm tube 1 becomes too small, the contact arm 61 moves in a clockwisedirection so that contact 63 is actuated, whereby the solenoid valves V1and V2 are closed and valves V3 and V4 are opened so that additional airis exhausted from the pressure chamber 3 by fan 65 which draws the airthrough line 54b and exhausts it through valve V4 in line 66. Thisreduces the pressure in chamber 3 so that the film tube can expand untilthe contact arm moves counterclockwise until it reaches thenonconducting portion of the switch 55 between the two contact plates 63and 64. At this point, there is no electrical contact with the resultthat valves V1 and V4 are opened and valves V2 and V3 are closed. On theother hand, if the diameter of the film tube becomes too large, thecontact arm 61 moves further in a counterclockwise direction to closethe circuit over contact plate 64, whereby valves V1 and V2 open andvalves V3 and V4 close. These positions of the valves cause additionalair to be drawn in from inlet line 40 by fan or blower 65 so that theair in storage chamber 41 is placed under greater compression and thepressure in chamber 3 is likewise increased so that the film tubediameter is decreased.

With reference to FIGS. 1 and 7, it can now be observed that all of theforegoing advantages and improvements with reference to the initialcontrolled inflation and the structure of the first stage or section ofthe elongated pipe are preserved while otherwise making minor changeswhich permit one to predominately stretch or orient the film tube ineither its longitudinal or transverse directions.

In order to stretch the film tube predominately in its longitudinaldirection, it is merely necessary to omit the heating chamber 8 (FIG. 1)so that the sleeve 28 with its shell 29 is connected directly to thethird section of the elongated pipe and has the same diameter, i.e.,sleeve 28 simply becomes an added extension or fourth section on thepipe 7. Thus, as shown in FIG. 7, the pipe sections 7a, 7b and 70 remainsubstantially the same while connecting on a new sleeve 7d of equaldiameter enclosed by shell 67 which is provided with an inlet conduit 68and outlet conduit 69 for the circulation of a fluid cooling medium. Inthis case, the film tube is treated as before in the first three stagesor sections of the elongated pipe 7 but is then cooled in section 7d.Since there is no second inflation chamber, it will be apparent that asthe film tube is drawn in the usual manner by the pinch rolls at a ratewhich exceeds the rate of extrusion, it will be stretched primarily in alongitudinal direction. The heated section 7c of the pipe is preferablyheated somewhat higher in this case as compared to its preheatingfunction with a subsequent heating and inflation chamber. Thus, it willbe understood that most of the lon gitudinal stretching occurs insection 7c where the braking pressure or tension exerted by section 7balso has a favorable effect. It is of course possible to achieve somedegree of transverse stretching in the pressure chamber 3, but this isquite slight in comparison to the high degree of longitudinal stretchobtained as the tubular film is drawn from heated section 7c throughcooled section 7d. After the longitudinal stretching and cooling, thefilm tube is flattened and collected in the usual manner.

Tubular films or foils which are predominately oriented in thetransverse direction can be produced by using the same apparatus asshown in FIG. 1, provided that the braking section 7b of the elongatedpipe 7 is either omitted entirely or is nullified by introducing airunder a slight excess pressure externally of the tubular film in section7b. This application of pressure is easily accomplished where section 7balready has a porous structure or is produced to contain a large numberof small holes or openings. Furthermore, under this application ofpressure rather than a vacuum, section 7b may still contain the textileliner 18 without creating a braking tension on the tube film. Inessence, by completely avoiding any braking effect, the elongated pipeconnecting the pressure chamber 3 and the heating chamber 8 is reducedto two treatment sections or stages instead of three, i.e., a firststage where the tubular film is cooled over at least the first portionthereof and a second stage where the film is preheated before it emergesinto the second inflation zone or heating chamber for transversestretching.

The tubular films obtained according to the various embodiments of theinvention do not always have a completely satisfactory stability duringstorage. For example, due to possible temperature fluctuations duringstorage or shipment, an after-shrinkage of the highly oriented films cancause distortions of the film and in extreme cases can even cause damageto the wound spool, especially with very large spools with long runninglengths of film stored thereon.

However, in those cases where the film tube stretched according to theinvention has a high capacity for after-shrinkage, its quality in thisrespect can be improved if the flattened tube emerging from thetransverse pinch or draw rollers is conducted over a plurality of heatedrollers and then over a cooled roller, the peripheral velocity of theseadditional rollers being, reduced in steps or stages from roller toroller to about percent below the peripheral velocity of the pinchrollers. Thus, as shown in FIG. 8, the film 1 after being flattened bythe pinch rollers 34 is conducted over the guide pin'or rod 70 and thenover four heated rollers 71, 72, 73 and 74 followed by a fifth cooledroller 75. The treated film is preferably contacted first on one sideand then the other in each successive roller.

The extent of the shrinkage and the number as well as the temperature ofthese additional rollerswill differ for different types of thermoplasticfilms and with variations in wall thickness, but optimum conditions canbe readily determined by simple tests. The gradation of velocity of theheated rolls is preferably adjusted in such a manner that the totalshrinkage is equally divided among them, wherein the first heated rollerto receive the flat tube can still have the same velocity as the pinchor draw rollers. The temperature of the heated rollers is advantageouslyabout 40 to 60 C. (See Example 7).

Through this treatment of the flattened tube prior to its collection onthe takeup spool, it receives a controlled aftershrinkage of up to about5 percent and does not then exhibit any further deformation duringstorage on rolls or spools. Also, films or foils which have beenobtained with this special treatment and then cut to provide a flatsheet have a surprisingly improved planar orientation.

The process of the invention is further illustrated by the followingexamples, it being understood that the invention is not limited to suchspecific process conditions. All percentages expressed in the examples,other than shrinkage values, are by weight. Examples l3 use theapparatus of FIG. 1.

EXAMPLE 1 A polyvinyl chloride obtained by suspension polymerization andstabilized with 1.5 percent dibutyl tin maleinate, with a 1(- value of65 and a plasticizer content of 25 percent dioctylphthalate, is blownwith a 60-fold extruder vertically downward through an annular die slothaving a 150 mm. diameter and a gap width of 700 microns at a throughputof 36 kg./hr. into a seamless tube 1. This tube enters the pressurechamber 3. The relay 6 regulates the compressed air of approximately 40mm. water column by control of the valves where air enters throughnozzles 4 and emerges through conduits 5, so that the maximum tubediameter at the sensor or feeler of the relay 6 amounts to 320 mm. 1 5mm. The first zone 7a of the pipe or passageway 7 is 340 mm. long andhas a diameter of 300 mm. The distance of its upper edge or rim 9 fromthe face of the die 2 is 300 mm. The inner wall of this first pipesection is roughened by sand jets to a roughening depth of 35 to 55microns and then abraded with sandpaper in order to remove any sharpburrs or edges possibly present so that the surface of the tubular filmcannot be harmed. There are no spiral grooves in the wall in this case.The cooling water enters at 12 C. into the shell space of this firstzone through line 10a and emerges at 10b. The second zone 7b is mm. longand its interior wall 20 is perforated with holes having a diameter of 1mm. Over the holes there is stretched a plush or piled fabric ofpolyester fibers. The vacuum of the shell space 21 is adjusted to 20 mm.water column below atmospheric pressure. The third zone 7c is 200 mm.long and has a slightly roughened surface with a roughening depth of 15to 20 microns. The inner wall of this third zone is heated to 65 C.,since at this temperature of the pipe wall and at a temperature of 92 C.in the upper portion and at 72 C. in the lower portion of the heatingchamber 8, which has an inside diameter of 800 mm. and a length of 400mm., the most favorable pear shape is formed on the tube. Thecalibrating sleeve 28, which is maintained at a temperature of 20 C.,has a diameter of 610 mm. and a length of 560 mm. The inside wall ofthis sleeve is smoothed or polished in order to preserve the surface ofthe tube. The wall thickness of the tube on entering the first zone 7ais 60 microns and that of the stretched tube on leaving the sleeve 28 is15 microns. The drawoff speed is 18 m./min. The roller system 32 withthe calibrating sleeve 28, the pinch rollers 34 and the takeup roll 35moves radially about the tube axis, alternating at 270 every threeminutes to the right and to the left.

The film obtained under these conditions has a shrinkage in longitudinaland transverse directions of 45 to 48 percent over the entire productionrun to complete a conventional roll of film. If the vacuum is notapplied in the second zone 7b, the longitudinal shrinkage is only 40percent. If zone 7b is removed completely, then the longitudinalshrinkage amounts to only about 25 percent.

EXAMPLE 2 In this test, there is used a copolymer of 90 percent vinylchloride and 10 percent vinyl acetate with the same annular die slot asin Example 1. The interior pressure of the tube 1 is 90 mm. and theexternal pressure around the tube in the pressure chamber 3 is 80 mm.(measured as mm. water column above atmospheric pressure), whereby amaximum diameter of the preinfiated tube of 315 i 5 mm. is achieved inchamber 3 and maintained or controlled by the relay 6. The slightlyroughened wall (about 20 microns) of the first pipe section or zone 7a,which is 300 mm. long and whose upper edge 9 is 290 mm. away from theface of the die 2, is subdivided by crossed spiral grooves 12 of 0.8 mm.in depth and 0.8 mm. width into rhombic wall surfaces of 5 mm. sidelength. At the end of this first zone, there is an annular groove orrecessed channel 14 with four outlet apertures or conduits 15 which areconnected with one another. Through these outlets, the small amount ofair coming from the pressure chamber 3 emerges in controlled manner overvalve 16, and this throughput of air serves for the formation of an aircushion around the tubular film.

The second pipe section or zone 7b corresponds to that of Example 1, butno vacuum is used. Also the third zone 7c is designed as in the previousexample, but it is heated to 60 C. The temperature in the upper portionof the heating chamber 8 is C. and that in the lower portion is 100 C.The smooth calibrating sleeve 28 with a diameter of 610 mm. and a lengthof 560 mm. is maintained at 40 C. With a draw speed of 9 m./min. thereis obtained a shrinkage film of 30 microns thickness, this film havingvery uniform longitudinal and transverse shrinkage values over theentire production run.

EXAMPLE 3 Using the same apparatus as in Example 1, there is extruded apolypropylene melt through an annular die slot of 700 microns gap widthand 80 mm. in diameter. The resulting tube 1 is maintained on relay andsensor 6 to a diameter of only 1 l0 mm. This is necessary, because thestretch factor of polypropylene of approximately 1:6 to 1:7 isconsiderably greater than that of polyvinyl chloride of only 1:2. Theinternal blowing pressure is 60 mm. and the external pressure in thechamber 3 is 50 mm. water column. The distance of the upper edge 9 ofpipe 7a from the face of the die 2 is mm.

The three zones of the pipe passage have an inside diameter of only 100mm., but their lengths are comparatively greater than in Example 1,namely: first zone 680 mm., second zone 200 mm. and third zone 400 mm.The temperature of the third zone is 130 C. Likewise the temperatures ofthe heating chamber above and below are higher than in Example 1, namely145 and 130 C., so that there is achieved a good biaxial stretching. Thecalibrating sleeve 28 with a diameter of 600 mm. and a length of 500 mm.has a temperature of 30 C. The draw speed is m./min. at an extrusionrate of 40 kg./hr. of polypropylene and at a desired film thickness of15 microns.

EXAMPLE 4 A stabilized, plasticized polyvinyl chloride granulate ofa 1(-value of 60 and having a plasticizer content of 25 percent is extrudedwith a 60-fold extruder through an annular die slot of 150 mm. diameterand 0.7 mm. gap space at a throughput of 40 kg./hr. into the pressurechamber 3 wherein the greatest diameter of the initially inflated tubeamounts to 390 mm. In the elongated pipe 7 which has a diameter of 360mm., the tube is treated in three stages:

first, in the double-walled cooling pipe 7a which has a length of 140mm. and inner wall surfaces containing spiral grooves of 6 mm. depthcrossing each other to form rhombic contacting surfaces whose sidesmeasured 5 mm. in length, the mantle or jacket permitting a cooling ofthe film tube;

next, in the braking section of the pipe 7b having a length of 200 mm.and also a double-wall jacket wherein the inner wall is perforated with1 mm. openings and is lined with a felt of 3 mm. thickness; and

finally in the stretching pipe section 7c which has a length of 200 mm.,the inner wall surfaces in this case having the same spiral grooves asin section 7a and being maintained at 75 C. by means of thermostats.

While these three stages or sections of the pipe 7 are generallyarranged as shown in FIG. 1, the apparatus in this case correspondsclosely to that shown in FIG. 7 where the socalled calibrating sleeve 7dis connected directly to the three previous sections of pipe 7 and hasthe same diameter as each of the preceding sections. The sleeve 7d is200 mm. long and has its inner wall surfaces roughened by sand jets to adepth of approximately 40 microns. The sleeve is cooled at a temperatureof 30 C. In this four stage treatment, the heating and inflating chamber8 of FIG. 1 is omitted and the calibrating sleeve 28 used in theprevious examples simply becomes an extension or fourth section of theelongated pipe 7, i.e., as indicated in FIG. 7.

After raising the pressure to 50 mm. water column in the interior of thefilm tube and with a vacuum of mm. water column in the jacket or shell21 of the braking section 7b, a foil of 15 microns thickness is obtainedwith a shrinkage value of percent in the longitudinal axis and only 8percent in the transverse axis.

By retarding the drawing-off speed to one-half ofits original value, a30 micron foil is obtained with the same shrinkage data; the interiorpressure on the tube increased in this case from the former mm. value upto 80 mm. (water column).

EXAMPLE 5 A stabilized copolymer of 90 percent vinyl chloride and 10percent vinyl acetate of the K-value of 60 is extruded at a throughputof 40 kg./hr. with a 60-fold extruder having an annular die slot of 150mm. diameter and a gap space of 0.8 mm. into the pressure chamber 3 toform a primary or initial tube of 30 microns thickness. The maximumdiameter of the tube in this pressure chamber amounts to 320 mm. Thefilm tube is led into the cooling pipe 7a (length 140 mm., diameter 300mm.), which is maintained at 50 C. The inner wall of the pipe section 70is provided with crossing grooves so as to yield rhombic panels orcontacting surfaces of 4 mm. length on each side and 0.8 mm. deep.Thereafter, the film tube runs through the pipe section 7b having aninner wall 17 produced from sintered metal and lined on the inner sidewith a velvet fabric composed of polyester fibers (See FIG. 4). Thespace 21 between the walls of pipe section 7b are kept under a pressureof 20 mm. water column. Next, the film tube is conducted into the pipesection 7c which is mm. long and maintained at 70 C., after which thetube passes into the heating chamber 8 (diameter= 800 mm., length 400mm.) which is heated by infrared radiation to C. in the upper portionand 100 C. in the lower portion. The stretched tube is then calibratedin a jacketed sleeve 8 of 600 mm. diameter at 40 C., and then wound upwith a film thickness of 15 microns. The film has a shrinkage value of55 percent in the transverse axis and 10 percent in the longitudinalaxis. The apparatus for this example of predominately orienting the filmin its transverse direction essentially corresponds to that shown inFIGS. 1-4.

EXAMPLE 6 The same procedure is followed as in Example 5 up to the pipesection 7a. Thereafter, the tubular film is conducted directly fromsection 7a into section 70 by omitting section 7b entirely. Thus, pipesection 7c becomes the second stage or treatment zone. The tubular filmis otherwise treated exactly as in Example 5. The shrinkage in thelongitudinal axis amounts to 15 percent in this case, while thetransverse shrinkage remains the same.

EXAMPLE 7 The film tube produced according to Example 2, while beingcollapsed and laid flat, leaves the pinch rolls 34 at a peripheralvelocity of the rolls of9 meters/minute. The film at this point is 30microns thick. Then the tube is conducted over four rollers heated to 50C. (See FIG. 8) The peripheral velocity of the rollers had been adjustedto 9 meters/min, 8.9 meters/min., 8.8 meters/min. and 8.7 meters/min.Then the flat tube is drawn off at a speed of 8.7 meters/min. on a pairof rollers of which one had been cooled to 12 C. There results a tubularfilm with a wall thickness of 33 microns which is highly stable duringstorage. The foils produced by cutting up this foil have an especiallygood planar condition.

While the process and apparatus of the invention are especiallyadvantageous for blowing films of polyvinyl chloride, vinyl chloridecopolymers and polyolefins such as polyethylene and polypropylene, theinvention is also adaptable and quite practical for film production fromother thermoplastic polymers. Exact processing conditions in terms ofthe degree of stretching and the temperatures required at various pointsin the process naturally depend on the type of thermoplastic material aswell as its extruded wall thickness and the amount of stretch impartedthereto. Also, in addition to plasticizers, the polymers may containother additives such as pigments, antioxidants and the like. Therefore,the invention is not limited to specific stretching values ortemperatures other than those which would be considered best suited toan individual thermoplastic material. The improvement achieved herein isinstead predicated upon the construction and arrangement of theapparatus or the particular processing techniques as defined by theappended claims.

The invention is hereby claimed as follows:

1. In an apparatus for extruding and biaxially stretching a tubularthermoplastic film including means to extrude the thermoplastic materialthrough an annular slot to form a seamless tube, means to inject aircentrally of said slot into said tube for inflation thereof, a pressurechamber surrounding said tube as it emerges from said annular slot withmeans to supply air into said pressure chamber externally of said tubeunder a controlled pressure which limits the inflation of said tube,means to heat and stretch said tube for orientation in at least one ofits longitudinal and transverse directions, means to collapse andflatten the stretched tube while it is drawn off and collected, theimprovement which comprises:

means to circulate cooling air under pressure through said pressurechamber externally of said tube; and

an elongated pipe connected to said pressure chamber with a diameter ofthe pipe being approximately -l0 percent smaller than the maximumdiameter of the inflated tube within said pressure chamber and said pipebeing arranged coaxially with the tube being drawn therethrough forpartial peripheral contact therewith in at least two successive sectionsincluding a. a first section in which the inner wall of the pipe has asurface roughness sufficient to substantially reduce the area of surfacecontact with said tube and means to cool the inner wall of said firstsection, and

b. a subsequent section in which the inner wall of the pipe also has asurface roughness sufficient to reduce surface contact with said tubeand means to heat the inner wall of said subsequent section.

2. An apparatus as claimed in claim 1 wherein the first section of thepipe extends into said pressure chamber with its annular rim positionedat a distance of not greater than approximately its own diameter fromthe point at which the thermoplastic material is extruded from theannular slot.

3. An apparatus as claimed in claim 1 wherein the surface roughness ofthe walls of said first section and said subsequent section of said pipecorrespond to a depth of roughness of at least 30 microns.

4. An apparatus as claimed in claim 1 wherein the inner wall of saidfirst section of said pipe contains grooves which provide a plurality ofchannels for air flow along the surface of said inner wall extendingfrom said pressure chamber to an annular collecting space in the innerwall of the pipe at the end of said first section, and at least oneregulatable release valve connected to said annular collecting space forwithdrawing air and regulating the flow of air through said channels.

5. An apparatus as claimed in claim 4 wherein the surfaces of the wallof the first section of said pipe between said grooves have a depth ofroughness of about to 30 microns.

6. An apparatus as claimed in claim 1 wherein the diameter of theelongated pipe is about 1.2 to 3 times the diameter of the annular dieslot.

7. An apparatus as claimed in claim l wherein said elongated pipe hasthree successive sections consisting of a. said first section,

b. a second section having means to exert a braking pressure on the tubepassing therethrough, and

c. said subsequent section, and wherein said means to heat and stretchthe tube includes a heating chamber connected to said pressure chamberby said elongated pipe and is adapted to permit inflation of the tubewith an accompanying transverse stretch thereon and pinch rolls arrangedto draw off the tube from said heating chamber at a rate sufficient toproduce a longitudinal stretch thereon.

8. An apparatus as claimed in claim 7 wherein the braking means of saidsecond section of the pipe includes a soft fibrous textile materialwhich lines at least a portion of the inner wall of the pipe and whichis arranged for frictional contact with the outer surface of the tubepassing therethrough.

9. An apparatus as claimed in claim 7 wherein the braking means of thesecond section of the pipe includes a permeable pipe wall over at leasta portion of its length and means to apply a vacuum through saidpermeable wall.

10. An apparatus as claimed in claim 9 wherein the braking means of saidsecond section of the pipe includes a soft fibrous textile materialwhich lines at least a portion of the inner wall of the pipe and whichis arranged for frictional contact with the outer surface of the tubepassing therethrough.

11. An apparatus as claimed in claim 7 wherein a calibrating sleeve isarranged to receive the inflated tube emerging from said heating chamberand means to adjust the temperature of the inner wall of saidcalibrating sleeve for fixation of an approximately uniform tubediameter.

12. An apparatus as claimed in claim 1 wherein a first gas collectingchamber is arranged to receive a feed line for the cooling air and tosupply said cooling air through a plurality of inlet openings into saidpressure chamber, a second gas collectin chamber is arranged to receivesaid cooling air from a plura ity of exit openings in said pressurechamber and to exhaust said cooling air to the atmosphere through adischarge line, and means responsive to variations in the maximumdiameter of the inflated film tube passing through said pressure chamberto supply additional air to said first gas collecting chamber when saidmaximum diameter increases above a preset value and to exhaustadditional air from said second gas collecting chamber when said maximumdiameter decreases below said preset value.

13. Apparatus as claimed in claim 12 wherein said means responsive tosaid variations in the maximum diameter of the inflated film' tubeincludes a blower in gaseous connection with both said feed line andsaid discharge line.

14. Apparatus as claimed in claim 12 wherein said means responsive tosaid variations in the maximum diameter of the inflated film tubeincludes an electrical switch operated by a feeler arm adapted tocontact said film tube at its point of maximum diameter.

15. Apparatus as claimed in claim l4wherein said switch is mountedwithin a capsule arranged within said pressure chamber at a positionexternally of the film tube, and said capsule contains inlet and outletmeans to direct a stream of air over said electrical switch.

16. Apparatus as claimed in claim 12 wherein said feed line to saidfirst gas collecting chamber is divided into one line feeding directlyto said first chamber and a second line cycled through said blowerbefore entering said first chamber, and said discharge line from saidsecond gas collecting chamber is divided into one line dischargingdirectly to the atmosphere and a second line cycled through said blowerbefore discharging to the atmosphere.

17. Apparatus as claimed in claim 1 wherein said elongated pipe has foursuccessive sections consisting of a. said first section,

b. a second section having means to exert a braking pressure on the tubepassing therethrough,

c. said subsequent section, and

d. a fourth section provided with means to cool the tube drawntherethrough, and wherein said pinch rolls are arranged to draw off thetube from said fourth section at a rate sufficient to produce apredominately longitudinal stretch thereon.

18. Apparatus as claimed in claim 1 wherein said elongated pipe has twosuccessive sections consisting of a. said first section, and

b. said subsequent section and wherein said means to heat and stretchthe tube includes a heating chamber connected to said pressure chamberby said elongated pipe and is adapted to permit inflation of the tubewith an accompanying predominately transverse stretch thereon.

fig UNHED STATES PATENT OFFICE QERTIFMATE @F CORRECTWN Patent No. 5, 5644 Dated March 21, 97

Inventor(s) Alfred Ebert et a1 it is certified that error appears intheabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 5, line 37, "1/3 percent and preferably not more than 1/2 per-"should read 3 percent and preferably not more than i 2 per- Signed andsealed this 3rd day of October 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTI'SCHALK Attesting OfficerCommissionerof Patents

1. In an apparatus for extruding and biaxially stretching a tubularthermoplastic film including means to extrude the thermoplastic materialthrough an annular slot to form a seamless tube, means to inject aircentrally of said slot into said tube for inflation thereof, a pressurechamber surrounding said tube as it emerges from said annular slot withmeans to supply air into said pressure chamber externally of said tubeunder a controlled pressure which limits the inflation of said tube,means to heat and stretch said tube for orientation in at least one ofits longitudinal and transverse directions, means to collapse andflatten the stretched tube while it is drawn off and collected, theimprovement which comprises: means to circulate cooling air underpressure through said pressure chamber externally of said tube; and anelongated pipe connected to said pressure chamber with a diameter of thepipe being approximately 5-10 percent smaller than the maximum diameterof the inflated tube within said pressure chamber and said pipe beingarranged coaxially with the tube being drawn therethrough for partialperipheral contact therewith in at least two successive sectionsinCluding a. a first section in which the inner wall of the pipe has asurface roughness sufficient to substantially reduce the area of surfacecontact with said tube and means to cool the inner wall of said firstsection, and b. a subsequent section in which the inner wall of the pipealso has a surface roughness sufficient to reduce surface contact withsaid tube and means to heat the inner wall of said subsequent section.2. An apparatus as claimed in claim 1 wherein the first section of thepipe extends into said pressure chamber with its annular rim positionedat a distance of not greater than approximately its own diameter fromthe point at which the thermoplastic material is extruded from theannular slot.
 3. An apparatus as claimed in claim 1 wherein the surfaceroughness of the walls of said first section and said subsequent sectionof said pipe correspond to a depth of roughness of at least 30 microns.4. An apparatus as claimed in claim 1 wherein the inner wall of saidfirst section of said pipe contains grooves which provide a plurality ofchannels for air flow along the surface of said inner wall extendingfrom said pressure chamber to an annular collecting space in the innerwall of the pipe at the end of said first section, and at least oneregulatable release valve connected to said annular collecting space forwithdrawing air and regulating the flow of air through said channels. 5.An apparatus as claimed in claim 4 wherein the surfaces of the wall ofthe first section of said pipe between said grooves have a depth ofroughness of about 10 to 30 microns.
 6. An apparatus as claimed in claim1 wherein the diameter of the elongated pipe is about 1.2 to 3 times thediameter of the annular die slot.
 7. An apparatus as claimed in claim 1wherein said elongated pipe has three successive sections consisting ofa. said first section, b. a second section having means to exert abraking pressure on the tube passing therethrough, and c. saidsubsequent section, and wherein said means to heat and stretch the tubeincludes a heating chamber connected to said pressure chamber by saidelongated pipe and is adapted to permit inflation of the tube with anaccompanying transverse stretch thereon and pinch rolls arranged to drawoff the tube from said heating chamber at a rate sufficient to produce alongitudinal stretch thereon.
 8. An apparatus as claimed in claim 7wherein the braking means of said second section of the pipe includes asoft fibrous textile material which lines at least a portion of theinner wall of the pipe and which is arranged for frictional contact withthe outer surface of the tube passing therethrough.
 9. An apparatus asclaimed in claim 7 wherein the braking means of the second section ofthe pipe includes a permeable pipe wall over at least a portion of itslength and means to apply a vacuum through said permeable wall.
 10. Anapparatus as claimed in claim 9 wherein the braking means of said secondsection of the pipe includes a soft fibrous textile material which linesat least a portion of the inner wall of the pipe and which is arrangedfor frictional contact with the outer surface of the tube passingtherethrough.
 11. An apparatus as claimed in claim 7 wherein acalibrating sleeve is arranged to receive the inflated tube emergingfrom said heating chamber and means to adjust the temperature of theinner wall of said calibrating sleeve for fixation of an approximatelyuniform tube diameter.
 12. An apparatus as claimed in claim 1 wherein afirst gas collecting chamber is arranged to receive a feed line for thecooling air and to supply said cooling air through a plurality of inletopenings into said pressure chamber, a second gas collecting chamber isarranged to receive said cooling air from a plurality of exit openingsin said pressure chamber and to exhaust said cooling air to theatmosphere through a discharge line, and means responsive to variationSin the maximum diameter of the inflated film tube passing through saidpressure chamber to supply additional air to said first gas collectingchamber when said maximum diameter increases above a preset value and toexhaust additional air from said second gas collecting chamber when saidmaximum diameter decreases below said preset value.
 13. Apparatus asclaimed in claim 12 wherein said means responsive to said variations inthe maximum diameter of the inflated film tube includes a blower ingaseous connection with both said feed line and said discharge line. 14.Apparatus as claimed in claim 12 wherein said means responsive to saidvariations in the maximum diameter of the inflated film tube includes anelectrical switch operated by a feeler arm adapted to contact said filmtube at its point of maximum diameter.
 15. Apparatus as claimed in claim14 wherein said switch is mounted within a capsule arranged within saidpressure chamber at a position externally of the film tube, and saidcapsule contains inlet and outlet means to direct a stream of air oversaid electrical switch.
 16. Apparatus as claimed in claim 12 whereinsaid feed line to said first gas collecting chamber is divided into oneline feeding directly to said first chamber and a second line cycledthrough said blower before entering said first chamber, and saiddischarge line from said second gas collecting chamber is divided intoone line discharging directly to the atmosphere and a second line cycledthrough said blower before discharging to the atmosphere.
 17. Apparatusas claimed in claim 1 wherein said elongated pipe has four successivesections consisting of a. said first section, b. a second section havingmeans to exert a braking pressure on the tube passing therethrough, c.said subsequent section, and d. a fourth section provided with means tocool the tube drawn therethrough, and wherein said pinch rolls arearranged to draw off the tube from said fourth section at a ratesufficient to produce a predominately longitudinal stretch thereon. 18.Apparatus as claimed in claim 1 wherein said elongated pipe has twosuccessive sections consisting of a. said first section, and b. saidsubsequent section and wherein said means to heat and stretch the tubeincludes a heating chamber connected to said pressure chamber by saidelongated pipe and is adapted to permit inflation of the tube with anaccompanying predominately transverse stretch thereon.